18 J. GHAHAM KERR. 



segmentation cavity. At its outer edge this mass of small 

 cells passes gradually into the large inner cells. The sum of 

 small cells in question is the rudiment of notochord and meso- 

 blast. It is perfectly continuous across the middle line, and 

 is separated from the cavity of the archenteron by a definite 

 archenteric roof composed of cells closely fitted together. 



The cells of the notochordal-mesoblastic rudimeut are the 

 small blastomeres which are seen in earlier stages lying 

 below the floor and round the edges of the segmentation 

 cavity, or penetrating that cavity as a sponge-work. 



A little later — in Stage 14 (cf. Text-fig. 1) — a transverse 

 section (PI. 3, fig. 15) exhibits very similar features, only 

 now the mesoblastic cells are in close contact with one 

 another, and the mesoblastic rudiment is found to be grow- 

 ing at its edges by delamination from the underlying large 

 yolk-cells. The rate of this growth varies much. As a 

 rule, in an egg of Stage 14 the mesoblast extends very little 

 below the level of the archenteron on each side, though 

 in one case I found that it had grown right round the ventral 

 side of the egg. The process is in any case usually com- 

 pleted by Stage 18 or 20. For example, in an egg of Stage 

 18 I find that, although the actual splitting off of the 

 mesoblast has taken place only to a level slightly below 

 that of the archenteron, the superficial layer of yolk has 

 become fine-grained all round the egg, and here and there a 

 small mesoblast cell has separated off the large yolk-cells 

 beneath. Such mesoblast cells are often split off far beyond 

 the edge of the sheet of continuous mesoblast, so that when 

 I speak of the mesoderm spreading over the hypoblast I must 

 guard against giving the impression that the sheet is neces- 

 sarily continuous up to a definite margin. Finally, in eggs of 

 Stage 21 the stratum containing fine-grained yolk has been 

 cut off the underlying hypoblast all over, as a definite layer, 

 somewhat irregular in places, of rounded mesoblast cells. 



Where the mesoblastic rudiment has in its early stages 

 largely developed intercellular spaces, the boundary be- 

 tween it and the lai'ge yolk-cells is sharply marked very 



